How can we utilize the alternative energy technology for unmanned aerial vehicles (UAVs)? When we think about the unmanned aerial vehicles (UAVs) we think of saving human resource, undoubtedly the most precious resource of any nation. But UAVs have some drawbacks too. They can’t fly to the far and distant places because after traveling certain distance an aircraft needs refueling. And here we face the disadvantage of UAVs. They can’t refuel. But if they utilize solar energy they can travel more and work more effectively in enemy’s territory. The Air Force is thinking of using Dye-sensitized solar cells (DSSCs) for unmanned aerial vehicles (UAVs) in future. Then they can fly for a longer duration of time without refueling.

This project is undertaken by the University of Washington’s Multidisciplinary University Research Initiative (MURI) team. The lead researcher Dr. Minoru Taya is working on airborne solar cells by utilizing a flexible film and a thin glass coating with transparent conductive electrodes. He has reached to the conclusion that DSSCs are able to grab photons and convert them into synthesized electrons that can harvest high photon energy. What does Taya think about his project? He shares his thoughts, “These kinds of solar cells have more specific power convergence efficiency (PCE), very clean energy and easy scalability to a larger skin area of the craft, as well as, low-temperature processing, which leads to lower costs overall.” Taya is hopeful that, “Some of these challenges will be overcome by the researchers working under this AFOSR MURI within the next two years. In order to make the DSSCs’ solar energy harvester transferable to the wings of an UAV, additional engineering tasks remain, which may require another project to be funded for five additional years.”

An unmanned aerial vehicle (UAV) is a remotely piloted aircraft. Today UAVs perform not only military (reconnaissance and attack) missions but are also used in a growing number of civil applications, such as firefighting, environment monitoring and are often preferred for other dirty or dangerous missions. The results gained in military researches were often used in civil applications and vice versa. So financing of this project, undertaken by the University of Washington’s team, by Air Force must be invited.

A dye-sensitized solar cell or DSSc that belong to the group of thin film solar cells, also known as Grätzel cell is based on a semiconductor formed between a photo-sensitized anode and an electrolyte. This element is extremely promising in many applications because it is made of low-cost materials and is very simple to manufacture in comparison with older solid-state cell designs. It can be engineered into flexible sheets and is mechanically robust. Although its conversion efficiency is less than the best thin-film cells, its price-performance ratio should be high enough to allow them to compete with conventional PV- devices. The overall quantum efficiency for green light is about 90%. DSSc's offer slightly higher V_oc than silicon, about 0.7 V compared to 0.6 V. The maximum possible photocurrent is determined by the overlap between solar flux spectrum and absorption spectrum of the sensitized TiO₂ layer, for comparison, a traditional silicon-based solar cell offers about 35 mA/cm², whereas current DSSc's offer about 20 mA/cm². Thus overall peak power production for DSSc's is about 11%.

Other thin-film technologies are typically around 8%, and traditional low-cost commercial silicon panels operate between 12% and 15%. DSSc's are therefore able to work under cloudy skies and non-direct sunlight. This makes DSSc's attractive as a replacement for existing technologies in applications like powering UAVs, where the mechanical robustness and light weight of the glass-less collector is a major advantage.

DSSc has higher efficiencies in higher temperatures and operates satisfactorily at lower internal temperatures.

The major disadvantage to the DSSc design is the use of the liquid electrolyte, which has temperature stability problems. Replacing the liquid electrolyte with a solid has been a major ongoing field of research. Recent experiments using solidified melted salts have shown some promise.

DSSc's are still at the start of their development cycle. One of the way of boosting efficiency gains include the use of quantum dots for conversion of higher-energy light into multiple electrons, using solid-state electrolytes for better temperature response, and changing the doping of the TiO₂ to better match it with the electrolyte being used.

Vasil Sidorov after www.alternativeenergy.com

18.07.2009

 E-mail: sidorovvasil@gmail.com